Phenol is an important product in the chemical industry, with utility in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, alkyl phenols, and plasticizers. Phenol is currently produced by the selective catalytic decomposition of cumene hydroperoxide although other alkylaromatic hydroperoxides, particularly cyclohexylbenzene hydroperoxide and sec-butylbenzene hydroperoxide, are attractive precursors in the production of phenol.
Current commercial processes for the cleavage of alkylaromatic hydroperoxides almost exclusively use sulfuric acid as the catalyst, even though this yields phenol selectivities of only 92% to 96% of the theoretical yield. In addition, of course, there are many disadvantages inherent in the use of sulfuric acid. Thus, sulfuric acid is corrosive, especially in the presence of water, potentially requiring expensive material for reactor construction. In addition, sulfuric acid needs to be neutralized before product separation, which involves the use of additional chemicals, such as phenates, caustic, and/or organic amines. Moreover, the salts generated by the neutralization require separation and disposal and generate waste water which needs to be treated. There is therefore significant incentive to replace sulfuric acid with a solid acid catalyst that can alleviate or eliminate these problems.
For example, U.S. Pat. No. 6,297,406 discloses a process for producing phenol and acetone from cumene hydroperoxide comprising the step of contacting cumene hydroperoxide with a solid-acid catalyst comprising a mixed oxide of cerium and a Group IVB metal, such as zirconium. A similar process is disclosed in U.S. Pat. No. 6,169,215 but using a solid-acid catalyst comprising a mixed oxide of a Group IVB metal and a Group VIB metal, such as chromium, molybdenum, and tungsten.
International Publication No. WO2010/042261 discloses that other alkyl aromatic hydroperoxides, such as cyclohexylbenzene hydroperoxide and sec-butylbenzene hydroperoxide, can be cleaved to produce phenol in the presence of a catalyst comprising an oxide of at least one metal from Groups 3 to 5 and Groups 7 to 14 of the Periodic Table of the Elements and an oxide of at least one metal from Group 6 of the Periodic Table of the Elements. Generally, the catalyst comprises an oxide of at least one metal from Group 4 of the Periodic Table of the Elements, such as zirconia, and an oxide of at least one metal from Group 6 of the Periodic Table of the Elements, such as an oxide of molybdenum and/or tungsten. In one embodiment, the catalyst further comprises an oxide of at least one metal from Groups 8 to 11 of the Periodic Table of the Elements, such as an oxide of iron and/or copper.
The conversion of dialkyl ethers to their corresponding alkenes and alkanols is also an important reaction in a number of commercial processes. Thus, for example, this reaction is used to remove ethers, such as isopropyl ether, produced as the by-products of other processes, such as the hydration of propylene to produce isopropanol. In addition, an important route for the production of tertiary olefins involves reaction of mixed olefins with an alcohol over an acid catalyst to selectively produce a tertiary alkyl ether, separation of the ether from the remaining olefin stream, and decomposition of the ether to the desired tertiary olefin. This latter process relies on the fact that tertiary olefins react with alcohols more rapidly than either secondary or primary olefins and hence provides an effective method for extracting tertiary olefins, such as isobutene and isoamylene, from a mixed olefin stream. For the purposes of this invention, a tertiary olefin or isoolefin will be understood to be an olefin containing at least one carbon atom that is covalently bonded to three other carbon atoms.
Current commercial processes for the selective decomposition of dialkyl ethers, such as methyl tert-butyl ether (MTBE), frequently employ a fluoride-treated clay, such as hydrofluoric acid (HF) treated attapulgite (HFA), as the catalyst. However, the relatively high operating temperatures required by the HFA catalyst tends to increase the concentration of impurities such as dimethyl ether (DME) and isobutane in the product, as well as promoting side reactions, for example, diisobutylene dehydrocyclization and isobutene oligomerization and polymerization, that lead to fouling of the catalyst. As a result, the cycle length of the HFA catalyst normally ranges from only a few weeks to 30 or more days, which is a major disadvantage in that the loss of catalyst activity results in considerable losses in production time and leads to high catalyst replacement and disposal costs. There is therefore significant interest in finding alternative catalysts for the selective decomposition of ethers.
For example, U.S. Pat. No. 7,102,037 discloses a process for selectively converting a dialkyl ether to the corresponding alkene and alkanol, the process comprising contacting a feed containing at least one dialkyl ether with a catalyst comprising an acidic mixed metal oxide having the following composition:XmYnZpOq where X is at least one metal selected from Group 4 of the Periodic Table of Elements, such as zirconium, Y is at least one metal selected from Group 3 (including the Lanthanides and Actinides) and Group 6 of the Periodic Table of Elements, such as chromium, molybdenum, or tungsten, and Z is at least one metal selected from Groups 7, 8, and 11 of the Periodic Table of Elements, such as iron, manganese, or copper; m, n, p, and q are the atomic ratios of their respective components and, when m is 1, n is from about 0.01 to about 0.75, p is from 0 to about 0.1, and q is the number of oxygen atoms necessary to satisfy the valence of the other components.
According to the present invention, it has now been found that a ternary metal oxide system based on metals of Groups 3, 4, and 6 of the Periodic Table provides a solid acid catalyst in which the acid site density and strength can be tuned by control of the composition and synthesis conditions of the catalyst. The resulting catalyst can be tuned to be both active and selective for the decomposition of organic oxygenates, such as alkylaromatic hydroperoxides and dialkyl ethers.